Abstract:

The present invention discloses a control circuit for a variable frequency
DC motor, the control circuit comprising: a controller, having a voltage
sensing input end and a control output end, wherein the control output
end is used to deliver an output signal according to the difference
between a threshold voltage and the voltage at the voltage sensing input
end; a transistor, having a first terminal, a second terminal and a third
terminal, wherein the first terminal is coupled to the voltage sensing
input end, and the second terminal is coupled to the variable frequency
DC motor, and the third terminal is coupled to a reference ground; and a
voltage divider, coupled between the second terminal and the third
terminal, used to generate a feedback voltage for the voltage sensing
input end; wherein the voltage at the second terminal is regulated
according to the threshold voltage.

Claims:

1. A control circuit for a variable frequency DC motor, wherein said
variable frequency DC motor has a positive terminal coupled to a DC
supply voltage and a negative terminal coupled to said control circuit,
said control circuit comprising:a controller, having a voltage sensing
input end and a control output end, wherein said control output end is
used to deliver an output signal according to the voltage difference
between a threshold voltage and the voltage at said voltage sensing input
end;a transistor, having a first terminal, a second terminal and a third
terminal, wherein said first terminal is coupled to said voltage sensing
input end, and said second terminal is coupled to said negative terminal
of said variable frequency DC motor, and said third terminal is coupled
to a reference ground; anda voltage divider, coupled between said second
terminal and said third terminal, used to generate a divided voltage of
the voltage at said second terminal to couple to said voltage sensing
input end;wherein the voltage at said second terminal is regulated
according to said threshold voltage.

2. The control circuit for a variable frequency DC motor as claim 1,
wherein said controller further comprises:a reference output end, used to
generate a reference voltage; anda threshold voltage input end, used for
setting said threshold voltage.

3. The control circuit for a variable frequency DC motor as claim 2,
further comprising:a first resistor, connected between said reference
output end and said threshold voltage input end; anda thermal resistor,
connected between said threshold voltage input end and said reference
ground.

4. The control circuit for a variable frequency DC motor as claim 3,
further comprising a second resistor, which is connected between said
control output end and said first terminal.

5. The control circuit for a variable frequency DC motor as claim 1,
wherein said voltage divider comprises:a third resistor, connected
between said second terminal and said voltage sensing input end; anda
fourth resistor, connected between said voltage sensing input end and
said reference ground.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to motor control circuits, and more
particularly to a control circuit for a variable frequency DC motor.

[0003]2. Description of the Related Art

[0004]In supplying power to a variable frequency DC motor, a control
circuit is needed to keep the rotation speed of the variable frequency DC
motor stable. The variable frequency DC motor, for example implemented in
a fan, can operate with different switching frequencies according to
different rotation speed requirements. Please refer to FIG. 1, which
shows the architecture of a prior art voltage control circuit for a
variable frequency DC motor. As shown in FIG. 1, the prior art voltage
control circuit comprises a controller 101, a resistor 102, a thermal
resistor 103, a resistor 104, a transistor 105 and a variable frequency
DC motor 106.

[0005]In the architecture, the controller 101 comprises a reference
voltage output end, a threshold voltage input end, a driving output end
and a voltage sensing input end, wherein the reference voltage output end
is used to provide a reference voltage Vref, and the driving output
end is used to generate a driving signal VG according to the
difference between a threshold voltage Vth at the threshold voltage
input end and a sensing voltage Vsen at the voltage sensing input
end.

[0006]The resistor 102 is connected between the reference voltage output
end and the threshold voltage input end, and the thermal resistor 103 is
connected between the threshold voltage input end and a reference ground.
The threshold voltage Vth is generated according to a voltage
division formula: Vth=Vref×(the resistance of the thermal
resistor 103)/(the resistance of the resistor 102+the resistance of the
thermal resistor 103), wherein the resistance of the thermal resistor 103
is a function of temperature, so that the threshold voltage Vth will
vary with temperature.

[0007]The resistor 104, connected between the controller 101 and the
transistor 105, is used to limit the base current of the transistor 105.
The transistor 105, having a base terminal connected to the resistor 104,
a collector terminal connected to a DC voltage Vdc and an emitter
terminal connected to the voltage sensing end, is used to operate in an
active region to provide the sensing voltage Vsen as a supply
voltage for the variable frequency DC motor 106.

[0008]The variable frequency DC motor 106 has an equivalent circuit as
shown in FIG. 3. The equivalent circuit comprises a resistor 301, a
capacitor 302 and switches 303˜306. The resistor 301 and the
capacitor 302 are used to supply a current IC to add with a current
IDC to generate a current IM for a motor M. The switches
303˜306 are used to provide alternative driving paths for the
supply voltage to drive the motor M, and the rotation speed of the motor
M will be increased if the supply voltage is raised up. Since the sensing
voltage Vsen, served as the supply voltage, is derived by
subtracting a voltage dropt across the transistor 105 from the DC voltage
Vdc, and the voltage dropt across the transistor 105 in the active
region actually occupies quite a portion of the DC voltage Vdc, the
highest level of the supply voltage of the variable frequency DC motor
106 is therefore limited and so is the rotation speed.

[0009]To increase the highest level of the supply voltage of the variable
frequency DC motor 106, a prior art current control circuit is proposed.
Please refer to FIG. 2, which shows the architecture of a prior art
current control circuit for a variable frequency DC motor. As shown in
FIG. 2, the prior art current control circuit comprises a controller 201,
a resistor 202, a thermal resistor 203, a resistor 204, a transistor 205,
a resistor 206 and a variable frequency DC motor 106.

[0010]In the architecture, the controller 201 comprises a reference
voltage output end, a threshold voltage input end, a driving output end
and a current sensing input end, wherein the reference voltage output end
is used to provide a reference voltage Vref, and the driving output
end is used to generate a driving signal VG according to the
difference between a threshold voltage Vth at the threshold voltage
input end and a current sensing signal Isen at the current sensing
input end.

[0011]The resistor 202 is connected between the reference voltage output
end and the threshold voltage input end, and the thermal resistor 203 is
connected between the threshold voltage input end and a reference ground.
The threshold voltage Vth is generated according to a voltage
division formula: Vth=Vref×(the resistance of the thermal
resistor 203)/(the resistance of the resistor 202+the resistance of the
thermal resistor 203), wherein the resistance of the thermal resistor 203
is a function of temperature, so that the threshold voltage Vth will
vary with temperature.

[0012]The resistor 204, connected between the controller 201 and the
transistor 205, is used to limit the base current of the transistor 205.
The transistor 205, having a base terminal connected to the resistor 204,
a collector terminal connected to the negative terminal of the variable
frequency DC motor 106 and an emitter terminal connected to the current
sensing end, is used to provide a driving current for the variable
frequency DC motor 106. The resistor 206 is used to carry the driving
current to exhibit the current sensing signal Isen.

[0013]The variable frequency DC motor 106, connected between a DC voltage
and the collector terminal, is driven by the driving current. According
to the equivalent circuit shown in FIG. 3, a driving current IDC is
added with a current IC to generate a current IM for a motor M.
When the switches 303˜306 are switching with a frequency, the
current IM and the current IC will exhibit periodic waveforms
shown in FIG. 4. Although the driving current IDC is regulated by
the current control circuit to a DC level, the voltage between the
positive terminal and the negative terminal will be varying due to the
current IC flowing through the resistor 301, and this will cause the
rotation speed of the motor M unstable.

[0014]Therefore, there is a need to provide a solution capable of
increasing the supply voltage of a DC motor and keeping the supply
voltage stable as well.

[0015]Seeing this bottleneck, the present invention proposes a novel
topology of a control circuit for a variable frequency DC motor.

SUMMARY OF THE INVENTION

[0016]One objective of the present invention is to provide a control
circuit for a variable frequency DC motor to operate in a wide bias
voltage range.

[0017]Another objective of the present invention is to provide a control
circuit for a variable frequency DC motor to operate in a stable manner.

[0018]Still another objective of the present invention is to provide a
control circuit for a variable frequency DC motor to operate adaptively
in response to temperature.

[0019]To achieve the foregoing objectives, the present invention provides
a control circuit for a variable frequency DC motor, wherein the variable
frequency DC motor has a positive terminal coupled to a DC supply voltage
and a negative terminal coupled to the control circuit, the control
circuit comprising: a controller, having a voltage sensing input end and
a control output end, wherein the control output end is used to deliver
an output signal according to the voltage difference between a threshold
voltage and the voltage at the voltage sensing input end; a transistor,
having a first terminal, a second terminal and a third terminal, wherein
the first terminal is coupled to the voltage sensing input end, and the
second terminal is coupled to the negative terminal of the variable
frequency DC motor, and the third terminal is coupled to a reference
ground; and a voltage divider, coupled between the second terminal and
the third terminal, used to generate a divided voltage of the voltage at
the second terminal to couple to the voltage sensing input end; wherein
the voltage at the second terminal is regulated according to the
threshold voltage.

[0020]To make it easier for our examiner to understand the objective of
the invention, its structure, innovative features, and performance, we
use a preferred embodiment together with the accompanying drawings for
the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is the architecture of a prior art voltage control circuit
for a variable frequency DC motor.

[0022]FIG. 2 is the architecture of a prior art current control circuit
for a variable frequency DC motor.

[0024]FIG. 4 is the current waveforms of IDC, IC and IM for
the circuit model in FIG. 3 when IDC is constant.

[0025]FIG. 5 is a control circuit for a variable frequency DC motor
according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026]The present invention will be described in more detail hereinafter
with reference to the accompanying drawings that show the preferred
embodiment of the invention.

[0027]Please refer to FIG. 5, which shows a control circuit for a variable
frequency DC motor according to a preferred embodiment of the present
invention. As shown in FIG. 5, the control circuit for a variable
frequency DC motor comprises a controller 501, a resistor 502, a thermal
resistor 503, a resistor 504, a transistor 505, two dividing resistors
506˜507 and a variable frequency DC motor 106.

[0028]In the architecture, the controller 501 comprises a reference
voltage output end, a threshold voltage input end, a driving output end
and a voltage sensing input end, wherein the reference voltage output end
is used to provide a reference voltage Vref, and the driving output
end is used to generate a driving signal VG according to the
difference between a threshold voltage Vth at the threshold voltage
input end and a sensing voltage Vsen at the voltage sensing input
end.

[0029]The resistor 502 is connected between the reference voltage output
end and the threshold voltage input end, and the thermal resistor 503 is
connected between the threshold voltage input end and a reference ground.
The threshold voltage Vth is generated according to a voltage
division formula: Vth=Vref×(the resistance of the thermal
resistor 503)/(the resistance of the resistor 502+the resistance of the
thermal resistor 503), wherein the resistance of the thermal resistor 503
is a function of temperature, so that the threshold voltage Vth will
vary with temperature.

[0030]The resistor 504, connected between the controller 501 and the
transistor 505, is used to limit the base current of the transistor 505.
The transistor 505, having a base terminal connected to the resistor 504,
a collector terminal connected to the variable frequency DC motor 106 and
an emitter terminal connected to a reference ground, is used to provide a
collector voltage to bias the variable frequency DC motor 106. The
transistor 505 can operate in an active region or a saturation region and
the collector voltage can be as low as ˜0.1 V.

[0031]The two dividing resistors 506˜507 connected between the
collector terminal and the emitter terminal of the transistor 505, are
used to provide a sensing voltage Vsen, which is proportional to the
collector voltage of the transistor 505, for the voltage sensing input
end of the controller 501. Through a negative feedback operation of the
architecture, the collector voltage of the transistor 505 is regulated
according to the threshold voltage Vth so that the difference
between the sensing voltage Vsen and the threshold voltage Vth
is approaching zero.

[0032]The variable frequency DC motor 106, connected between a DC voltage
Vdc and the collector terminal, is driven by the voltage difference
between the DC voltage Vdc and the collector voltage. As the sensing
voltage Vsen is derived from a divided voltage of the collector
voltage and the emitter terminal of the transistor 505 is connected to
the reference ground, the collector voltage can be controlled to a very
low voltage and the variable frequency DC motor 106 can therefore stably
operate in a wider range of bias voltage than in the prior art.

[0033]Through the implementation of the present invention, a novel control
circuit for a variable frequency DC motor is presented. The topology of
the present invention can provide a stable and wide range of bias voltage
for the variable frequency DC motor, so it does conquer the disadvantages
of prior art circuits.

[0034]While the invention has been described by way of example and in
terms of a preferred embodiment, it is to be understood that the
invention is not limited thereto. To the contrary, it is intended to
cover various modifications and similar arrangements and procedures, and
the scope of the appended claims therefore should be accorded the
broadest interpretation so as to encompass all such modifications and
similar arrangements and procedures.

[0035]In summation of the above description, the present invention herein
enhances the performance than the conventional structure and further
complies with the patent application requirements and is submitted to the
Patent and Trademark Office for review and granting of the commensurate
patent rights.